It is known that, in a packet-switched communication network, data are transmitted in the form of packets that are routed from a source node to a destination node through possible intermediate nodes. Each packet typically has a header and a payload. The header generally includes information allowing routing of the packet, such as the source node address and the destination node address. On the other hand, the payload generally comprises a portion of the data to be transmitted from the source node to the destination node. Exemplary packet-switched networks are Local Area Networks (e.g. Ethernet) and Geographic Area Networks (e.g. Internet).
On the other hand, in circuit-switched networks data are transmitted in the form of continuous bit flows carried from the source node to the destination node within plesiochronous or synchronous frames. Exemplary circuit-switched networks are PDH, SDH, Sonet and OTN networks.
Herein after, the expression “data unit” will designate a portion of a data flow transmitted in a communication network. In particular, in case of a packet-switched network, a data unit may be a packet or a portion of a packet. Besides, in case of a circuit-switched network, a data unit may be a plesiochronous frame, a portion of a plesiochronous frame, a synchronous frame or a portion of a synchronous frame.
Typically, a data unit is transmitted at a transmission time by the source node and is received at a reception time by the destination node. The time elapsing between transmission time and reception time is typically called “one way delay” (or, briefly, “delay”). The delay of a data unit is therefore given by the following equation:D(i)=Ri−Si,  [1 ]where Si is the transmission time and Ri is the reception time of the data unit.
The delay of a data unit mainly depends on the number of possible intermediate nodes crossed by the data unit from source to destination and on the permanence time of the data unit at the source node and at each possible intermediate node.
In a circuit-switched network, paths along which data units are transmitted are provisioned a priori by a network operator. Both the number of possible intermediate nodes crossed by data units and the permanence time of data units at each node are therefore determined a priori. Accordingly, the delay of a data unit is predictable, and all the data units of a same data flow have a same delay.
On the other hand, in a packet-switched network, data units are routed hop-by-hop by each node. Both the number of possible intermediate nodes crossed by data units and the permanence time of data units at each node are therefore unpredictable. Accordingly, the delay of a data unit is almost unpredictable. Besides, data units of a same data flow may have different delays.
In a packet-switched communication network, the difference in the delays of two data units (i.e. packets) of a same data flow is termed “interarrival jitter”. In particular, if Si and Sj are the transmission times for a first packet i and a second packet j, and Ri and Rj are the reception times for the first packet i and the second packet j, the interarrival jitter may be expressed as:J(i,,j)=(Rj−Ri)−(Sj−Si).  [2]
When a communication service (for instance, a voice or data service) is provided by means of a communication network, the delay and interarrival jitter of the data flows carrying the service strongly affect the Quality of Service perceived by the end users of the service. In particular, in case of real-time interactive services (calls, conference calls, video conferences, etc.), a high delay or high interarrival jitter may severely impair the Quality of Service perceived by the end users. Therefore, measuring the delay and interarrival jitters of the data flows carrying these types of services is of particular interest for network operators.
WO 84/00268 discloses a method for the determination of the time delay incurred by a packet as it progresses through a packet switching system, wherein each packet has a field for accumulating the total time delay incurred by the packet in the progressing through the switching networks of the packet switching system. The total time delay field is updated as the packet is routed through each switching network of the packet system.
EP 0 234 860 discloses an arrangement for determining the random delay experienced by packets as they progress through the transmission and/or switching networks. A network node entry timestamp function inserts a so-called packet originate time value into a single timestamp field of each packet header. An exit timestamp function inserts an updated timestamp value into the timestamp field of the packet header in place of the packet originate time upon the packet exiting the network node.